In the art of chemical pulping of comminuted cellulosic fibrous material, for example wood chips, the cellulose material is typically treated with cooking chemicals under pressure and temperature in one or more cylindrical vessels, known as digesters. This treatment can be performed continuously or in a batch mode. In the continuous mode, chips are continuously fed into one end of a continuous digester, treated, and continuously discharged from the other end. In the batch method, one or more batch digesters are filled with chips and cooking chemical, capped and then treatment commences. Once the treatment is finished the contents of the batch digester are discharged. In either batch or continuous digesters, a slurry of comminuted cellulosic fibrous material and cooking chemical is treated in one or more a cylindrical vessels.
In both continuous and batch digesters, in order to uniformly distribute both temperature and cooking chemical, cooking liquor is typically circulated through the slurry of chips and liquor, typically referred to as xe2x80x9cthe chip columnxe2x80x9d. This circulation is typically effected by some form of screen, located along the internal surface of the cylindrical vessel, a pump, a heater, and a return conduit. The screen retains the material within the digester as the liquor is removed, augmented with other liquors and/or a portion thereof removed, pressurized, heated, and then returned to the slurry in the vicinity of the screen or elsewhere. The proper operation of the digester and the production of uniform product having the best properties, for example, strength, are highly dependent upon the efficiency of this liquid circulation process.
The radial removal of liquor typically produces radial compression of the chip column in the vicinity of the screen assembly. In addition, the weight of the column of chips above the chips near the screen introduces another source of compression of the chips. Furthermore, the vertical movement of free liquor in the chip column, either upward or downward, can vary the compression load, or compaction, of the chip column. It is known in the art that this radial and vertical compression can interfere with the uniform movement of the chip column, which is so essential for the uniform treatment of the chips. For this reason, conventional digesters and screen assemblies are designed so that the diameter of the flow path increases just below the screen. This increase in diameter or xe2x80x9cstep-outxe2x80x9d relieves the compression in the chip column and permits more uniform movement of the column. This step-out typically consists of a radial increase of about 6 inches to 2 feet. Copending application Ser. No. 08/936,047 filed on Sep. 23, 1997 discloses several novel methods of accommodating this xe2x80x9ccolumn reliefxe2x80x9d while maintaining a relatively uniform vessel shell diameter.
The radial compression of the chip column against the surface of the screen, due to the radial flow of liquid, also aids in reducing pluggage of the screen surface. For instance, the normal pressure load on the screen surface in conjunction with the downward movement of the chip column acts to scour or xe2x80x9crubxe2x80x9d the surface of the screen. This xe2x80x9crubbingxe2x80x9d action helps to keep the apertures of the screen free of obstructions, for example, chips, pulp, or other debris. For instance, for vertically-oriented bar-type screens, the vertical rubbing action helps to dislodge any chips that may accumulate between the screen bars.
However, excessive radial compression can interfere with the uniform removal of liquid through the screen. As the radial flow increases, the compression induced by the flow in the chip column can compact the chip column making it difficult to pass liquid through the column. Therefore, the flow of liquid required to uniformly distribute chemical and temperature is typically limited. Thus, though some radial compression which produces a normal pressure on the surface of the screen is desired, this radial compression cannot exceed the compression that reduces the radial flow of liquid or hinders the axial flow of the chip column.
Typically, prior art screen assemblies comprise right cylindrical screen surfaces of relatively uniform diameter. These screen surfaces may comprise or consist of perforated plate, having slots or holes, or parallel-bar type construction having parallel apertures between the bars. These bars typically have a vertical orientation, but that may have various orientations including horizontal or at some oblique angle, for example, at a 45xc2x0 angle to the vertical.
By analyzing the distribution of the forces within the chip column that are produced by the flow of liquid within the column it has been found according to the invention that by designing these screen assemblies so that they are not uniformly cylindrical, but slightly divergent, the radial compression loading in the chip column and on the screen surface can be reduced, and the volume and rate of liquid that can flow through the chip column and be removed through the screen can be increased. For example, screen surfaces having a slight increase in diameter in the direction of chip flow can reduce the compression load in the chip column and improve the performance of the screen assembly, and the digester in general.
Conical divergent screening surfaces are not unknown in the art of chemical pulping. For example, continuous hydraulic digesters typically include a conical screen surface at the very top of the digester, in the vicinity of where the slurry of cellulose is introduced, to remove excess liquid from the slurry and return it the digester feed system. In two vessel digester systems those screens, which are typically referred to as xe2x80x9cbottom circulationxe2x80x9d or xe2x80x9cBCxe2x80x9d screens, are conical in shape but to do not provide the function of the screens of the present invention. Since BC screens are typically located above the chip pile they do not experience the compressive loading that screens located lower down in the digester do. Also, the BC screens typically do not interfere with the movement of the chip slurry through the digester. Conical BC screens would not be used in the cooking or extraction zones as are the screens of the present invention.
One embodiment of this invention comprises or consists of a cylindrical screen assembly for removing liquid from a slurry of comminuted cellulosic fibrous material in a cylindrical vessel having a diameter that diverges in the direction of the of the movement of the slurry. This screen assembly can have an angle of divergence of between about 0.5 and 45xc2x0. However, it is believed that using angles of divergence greater than approximately 15xc2x0 will diminish the rubbing action of the chip column on the screens that is desirable to prevent screen plugging. Though screens having larger angles of divergence can be used, it is preferred that the angle of divergence of the screen be limited to between about 0.5 and 10xc2x0, preferably, about 0.5 to 5xc2x0, to ensure that at least some form of normal rubbing force is exerted on the screen surface.
According to another aspect of the invention an assembly, per se, for use in screening liquid is provided. The assembly comprises: An annular screen assembly for separating liquid from solid material, the screen assembly having a screen surface with a top, a bottom, and a first internal diameter and a second internal diameter, and the screen assembly having an external diameter defining an annular volume exteriorly of the screen surface. The screen surface may comprise a substantially continuous cylindrical screen surface, or have a wide variety of other configurations as is conventional for screen surfaces per se, particularly for screens in chemical pulp digesters, and has a substantially constant opening size, and percentage of open area, in the flow direction.
According to another aspect of the present invention a method of treating a liquid slurry of comminuted cellulosic fibrous material under cooking conditions in a substantially vertical continuous digester having at least one substantially annular screen surface, and having a top and a bottom, to produce chemical pulp, is provided. The method comprises the steps of substantially continuously: (a) Introducing the slurry of comminuted cellulosic fibrous material into the digester adjacent the top thereof, to flow downwardly in the digester in a first flow path, having a first diameter. (b) Screening the slurry to remove liquid therefrom using the at least one screen surface, having a substantially constant screen surface opening size, and percentage of open area, in the first flow path. (c) During step (b) causing the slurry of comminuted cellulosic fibrous material to transition from the first flow path to a diverging second flow path [preferably having an initial second diameter, equal to or greater than the first diameter]. And, (d) removing the chemical pulp from adjacent the bottom of the digester.
The method also preferably comprises the further step (e), after step (c) and before step (d), of causing the downwardly moving slurry to move in a third flow path having a diameter substantially equal to or larger than the second diameter. There is also preferably the further step of repeating steps (b), (c), and (e), at least once prior to step (d), and there is the further step of heating the liquid removed in the practice of step (c), and reintroducing the heated liquid into the digester adjacent where it was removed. As is conventional, some of the liquid flow may be removed, and/or other liquid added, prior to return to the digester.
The invention also comprises a comminuted cellulosic fibrous material treatment vessel assembly. The vessel includes the following components: A substantially vertical vessel having a top, a bottom, an inlet and an outlet, and through which comminuted cellulosic fibrous material flows in a flow direction, the vessel having a substantially cylindrical wall, and preferably with at least one diameter-changing transition between the inlet and outlet. And, a screen assembly (e.g. provided at or just past the transition), the screen assembly comprising an annular screen surface diverging in the flow direction of the comminuted cellulosic fibrous material, and engaging (contacting) the slurry, so as to reduce the radial compression of material thereon [and also preferably to increase the volume and rate of liquid that can flow through the material and be removed through the screen surface compared to a right cylindrical surface of the same construction], the screen assembly having screen surface openings with a substantially constant screen surface opening size [and preferably percentage of open area] in the flow direction.
Typically the outlet is adjacent the bottom of the vessel and the inlet is adjacent the top so that the screen surface diverges downwardly. The screen surface preferably diverges at a substantially constant angle to the vertical of between about 0.5-10xc2x0, most preferably between about 0.5-5xc2x0. The screen surface may comprise a first screen surface, and the vessel may further comprise a second annular screen surface substantially immediately downstream of the first screen surface in the direction of flow, the second surface also diverging in the flow direction (at the same angles as indicated above). Preferably the annular screen surfaces are continuous, however they can be xe2x80x9ccheckerboardxe2x80x9d in configuration, or have other known configurations. The screen surfaces may have any conventional construction, such as perforated plate, bar, etc. For example, the screen surface comprises a perforated metal surface with perforations of substantially uniform size and density. As another example the screen surface comprises a plurality of bars spaced from each other in a direction substantially parallel to the flow direction, the spacing between the bars being substantially constant both from bar to bar and along the entire lengths thereof in the flow direction.
The invention also relates to a method of treating a liquid slurry of comminuted cellulosic material in a substantially vertical vessel (and preferably having at least one diameter transition). The method comprises the steps of: (a) Introducing the slurry into the vessel to flow substantially vertically therein in a flow direction. (b) while the slurry is flowing in the flow direction screening (e.g. at or just downstream of the diameter transition) the slurry to remove liquid therefrom while causing the liquid to diverge in the flow direction at an angle of between about 0.5-10xc2x0 using the at least one screen surface, having a substantially constant screen surface opening size [and preferably percentage of open area] in the first flow path. And, (c) downstream of step (b) in the flow direction, removing the slurry from the vessel. Steps (a) through (c) may be practiced substantially continuously, and so that the flow direction is substantially downward. Also there may be the further step of repeating step (b) at least once prior to the practice of step (c).
It is the primary object of the present invention to provide a simplified screen assembly for a comminuted cellulosic fibrous material treatment vessel which allows for increased liquid removal and improved material movement through the vessel. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings and from the appended claims.